This invention relates generally to radio frequency signal direction finding systems and more particularly to apparatus used in such systems for determining the angle of arrival of a radio frequency signal using multi-beam radio frequency antennas when such signal is a weaker radio frequency signal received subsequent to reception of a stronger, on-going, radio frequency signal.
As is known in the art, it is often desirable to determine the direction or angle of arrival of a source of radio frequency signals. One technique used to determine such direction is through the use of a multi-beam array antenna. As is known, such array antenna may be designed so that it produces a plurality of simultaneously existing beams of radio frequency energy, each one of such beams having the gain and bandwidth of the entire antenna aperture. One such multi-beam array antenna is described in U.S. Pat. No. 3,761,936 issued Sept. 25, 1975, "Multi-beam Array Antenna," inventors Donald H. Archer, Robert J. Prickett and Curtis P. Hartwig, and assigned to the same assignee as the present invention. Such antenna includes an array of antenna elements coupled to a microwave lens through constrained electrical paths. A desired number of simultaneous beams may be obtained, the constrained electrical paths and the microwave lens equalizing the time delay of received energy between a given one of a number of feed ports and all points on a corresponding wavefront of the received energy. The relative levels of the energy received at the feed ports are related to the direction, or angle of arrival, of the source of the received radio frequency energy.
One technique used to determine the angle of arrival of the received radio frequency energy using a multi-beam array antenna of the type described above has been to pass the energy received at each of the feed ports through separate receiver channels, convert such received signals to digital words representative of the levels of the energy of the received signals and then digitally compute the angle of arrival of the received signal by comparing the relative digitized signals representative of the levels of the energy received at the feed ports. That is, because each feed port is associated with a particular angle of arrival, detection of the feed port which receives the greatest amount of energy provides, to a first approximation, a determination of the angle of arrival of the received signal. Interpolation techniques between the levels of the energy at adjacent feed ports provides a more accurate indication of the angle of arrival of the received energy.
While the technique described above is adequate in the determination of the angle of arrival of a single received signal, such technique may not be adequate where a signal is received subsequent to another, still on-going, received signal. For example, if a first, relatively strong signal is received the energy at a first feed port associated with the angle of arrival of such first received signal will be relatively large, while the energy at the remaining feed ports willl be relatively small. If a second, much weaker signal having a different angle of arrival is received while the first, stronger signal is still being received the level of the energy at the first feed port will remain relatively large compared to the energy received at the remaining feed ports, even the feed port associated with the angle of arrival of the second received signal. Consequently, with the technique described above the system will continue to indicate the angle of arrival of the first received, on-going signal when the second, weaker signal is received and the angle of arrival of the second, weaker signal will not be determined.